The most common type of Mary Sue is a character based an the author's idealization of themself. Furthermore, because the author is imagining a preferred version of themself, and because faults are overlooked in favor of optimization, a Mary Sue tends to have only superficial resemblance to the author, sharing similar likes/dislikes and a similar spirituality (when applicable), but objectifying things such as worldview and relationships. This type of Mary Sue is more common simply because it's easier to write and is more appealing to the author. This Mary Sue is found in fanfics and original fiction alike. Yes, Mary Sues abound even in professional writing.

The other type of Mary Sue is a character intended to be an ideal match for another, appearing almost exclusively in fanfics. In this case, "ideal match" means that the character's positive traits are exaggerated to render impossible any competition for the love interest. Arguably more pernicious than the "self-idealization" type, the "ideal match" type by its very nature prevents compelling character or plot development, which the "self-ideal" Mary Sue may be able to avoid.
Self-idealization: "That fic was ridiculous. I could overlook the atrocious grammar, but not such an obvious and annoying Mary Sue. What's the point of releasing a story to the unsuspecting public if it's only written for the masturbation of the author's ego?"

Ideal match: "It's hard to find any other story that's so dull as one that has an ideal match Mary Sue in it. What is there to be interested in if the pair never has any trials to overcome?"
by Aesi January 30, 2008
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An original character (fem.) in fanfic or an original story, usually on the internet, who is far superior to all other characters. She is typically beautiful, intelligent, kind, and in all other ways "perfect". She usually serves as an important part in a pivotal plot element (ie: a prophecy) and becomes romantically involved with the author's favourite character in the story. The internet fiction world runs rampant with these characters.
dracosluvur: My character's name is Alienne Raven Jewel Sassandra Moonflower Romance Shimmer Honeybeam, she came to Hogwarts after being separated from her family in the U.S. and sold into illegal slavery in Japan, she's fluent in both languages and is part Fae. She has snow-white skin, black hair and sparkling eyes that change colour. She's tall, thin and busty, she fills in as the Charms professor sometimes and doesn't really have a house because she's just that unsortable so she has her own luxury room in one of the towers. Almost the entire male population adores her, staff and students, and she gets perfect marks and it figures that she is the one who actually kills voldemort.

SatiricalBanshee: AGH!DUCK AND COVER! MARY-SUE!

by SatiricalBanshee February 27, 2008
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Used to negatively describe a female character, usually in a fanfiction (but sometimes in a Tv show or movie,) that can be just be very annoying because they are too perfect or there aren't any/many weaknesses in their personality. If they are a side character, they can sometimes take the spotlight. If they are a main character, it can make an uninteresting story. Sometimes turn into a joke. Also look at Marty-Stu.
This is a prime example of a Mary-Sue that I found on a writers page on fanfiction.

"Michelle Isabella Fenton

Danny Phantom's thirteen-year-old sister, who is two years younger than him. She is Danielle Phantom's identical twin and Nat Wolff's girlfriend. She is a Harry Potter style witch and a Aquamarine style mermaid. Michelle has dark brown hair and aquamarine blue eyes. She is an Avatar style water and airbender. Michelle is 25 percent Twilight vampire, which explains why people think she looks perfect. Her hair is elbow-length and she is 5'3. In her power form, she has a white bikini top, full white knee-length skirt, a diamond on the very center or her forehead near her eyes. In this form, Michelle wears silver sandal heels. Her hair had white streaks in it. Two streaks of hair that are about 3 inches wide are curled and pinned at the center of the back of her head, towards the nape of her neck. Michelle is also very good with arrows.

Personality: Sweet and innocent. Very loving. Dispite how it may seem like an act, Michelle's personality is pure. Can be a bit protective at times, but only for the better.
Aliases: Lena Yue"

Doesn't that character sound kinda annoying?
by Nobody That Important July 14, 2008
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A girl who thinks she's so pretty and perfect; Girl version of Marty Stu.
Julia: omg girls, Alma is such a Mary Sue
Emma: I know right. She's the reason who Julian broke up with Ingrid
Sophie: Oh no, here she comes. Quick, act natural
Alma: Hi girls, like my hair? It's better than yours
Sophie (muttering): What a Mary Sue
by Betty Jo October 14, 2018
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The female protagonist of a story.

(Archaic) A female character, usually an authorial self-insert in a fan fiction, that is annoyingly perfect. Often unique in some implausible way, any problems they face are typically intended to make them seem tragic or emotionally deep, rather than complex or flawed.
"I can't believe you like that, the main character is such a mary sue."
by monkmunk April 11, 2015
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A character too perfect for their setting. Most often, this character is talented and attractive, and anyone who doesn't adore them is portrayed as mean, stupid, or evil. It's common for them to be the smartest, even if this requires everyone else to act stupider than they should. Out of place but awesome names are also frequent occurrences. They lead charmed lives, and any conflict or drama they are met with will be either overdone to the point of headache induction, or easily brushed aside.

While not always the case, Mary Sues are more often written by someone who is just starting out, either by someone who wrote their own personal power fantasy and expects everyone else to applaud them for it, or by someone using shortcuts to try and make their character impressive to others.

These characters are considered a mark of poor writing because they give too much favoritism to the character in question, and it comes off to readers as the author trying to artificially make their character the best around through shortcuts rather than well crafted development. Their perfectness also tends to prevent the stories they're in from developing suspense, making not only the character but the entire work they're in boring.

While the term Mary Sue, along with the male version Gary Stu, has been applied to both fan characters and canon characters in published works, it is much more common to see used on fan characters. Particularly those in fan fiction, which are most often female.
So this girl's name is Crystal Roseblade, she grew up in the same orphanage as the protagonist and was a childhood friend of his, but he forgot about her, and now she's come back out of nowhere as a huge pop idol who's secretly the world's greatest demon slayer? Do you think you might be writing a Mary Sue?
by ring of fates May 21, 2014
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When a girl (preferably Mary Sue Coleman) watches you while you sleep and you suddenly jizz in her face and yell 'SURPRISE' !!
Everyone heard Chris yell 'surprise' when he Mary Sue Surprised the president of the University of Michigan last night..
by kelseykilla October 31, 2010
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